An organic light-emitting device, also referred to as an organic electroluminescent device, can be constructed by sandwiching two or more organic layers between first and second electrodes.
In a passive matrix organic light-emitting device (OLED) of conventional construction, a plurality of laterally spaced light-transmissive anodes, for example indium-tin-oxide (ITO) anodes, are formed as first electrodes on a light-transmissive substrate such as, for example, a glass substrate. Two or more organic layers are then formed successively by vapor deposition of respective organic materials from respective sources, within a chamber held at reduced pressure, typically less than 10−3 Torr (1.33×10−1 Pa). In addition to doped or undoped organic light-emitting material, typical organic layers used in making OLEDS are doped or undoped organic hole-injecting material, doped or undoped organic hole-transporting material, and doped or undoped organic electron-transporting material, where doping refers to adding a minor constituent to enhance the electrical performance, optical performance, stability, or lifetime of a given material or device constructed thereof. A plurality of laterally spaced cathodes is deposited as second electrodes over an uppermost one of the organic layers. The cathodes are oriented at an angle, typically at a right angle, with respect to the anodes.
Applying an electrical potential (also referred to as a drive voltage) operates such conventional passive matrix organic light-emitting devices between appropriate columns (anodes) and, sequentially, each row (cathode). When a cathode is biased negatively with respect to an anode, light is emitted from a pixel defined by an overlap area of the cathode and the anode, and emitted light reaches an observer through the anode and the substrate.
In an active matrix organic light-emitting device (OLED), an array of anodes is provided as first electrodes by thin-film transistors (TFTs), which are connected to a respective light-transmissive portion. Two or more organic layers are formed successively by vapor deposition in a manner substantially equivalent to the construction of the aforementioned passive matrix device. A common cathode is deposited as a second electrode over an uppermost one of the organic layers. The construction and function of an active matrix organic light-emitting device is described in U.S. Pat. No. 5,550,066, the disclosure of which is herein incorporated by reference.
Organic materials, thicknesses of vapor-deposited organic layers, and layer configurations, useful in constructing an organic light-emitting device, are described, for example, in U.S. Pat. Nos. 4,356,429, 4,539,507, 4,720,432, and 4,769,292, the disclosures of which are herein incorporated by reference.
Other kinds of imaging devices such as imaging phosphors for computed radiography and x-ray photoconductive devices for digital radiography depend on the ability to coat the active materials uniformly over large areas.
For sufficiently small substrates, a point source approach can be implemented wherein the material to be deposited emanates from a localized heated crucible and the substrate is placed sufficiently far from the localized region of vaporization that the coating is sufficiently uniform along the substrate. As substrate size increases or working distance decreases, rotary or planetary motion of the substrate relative to the localized source is often required to produce the desired uniformity.
By elongating the vaporization source and providing for translation of source and substrate relative to one another, the desired uniformity can be attained at considerably smaller working distances, and thus considerably higher rates and better materials utilization, if desired. Scaling of such a process to large areas (i.e. substrates greater than 15 cm in at least one dimension) is considerably easier than for point sources.
An elongated source for thermal physical vapor deposition of organic layers onto a structure for making an organic light-emitting device has been disclosed by Robert G. Spahn in commonly assigned U.S. Pat. No. 6,237,529. The source disclosed by Spahn includes a housing, which defines an enclosure for receiving solid organic material, which can be vaporized. The housing is further defined by a top plate, which defines a vapor efflux slit-aperture for permitting vaporized organic materials to pass through the slit onto a surface of a structure. The housing defining the enclosure is connected to the top plate. The source disclosed by Spahn further includes a conductive baffle member attached to the top plate. This baffle member prevents particles of organic material from passing through the slit in the top plate by preventing line-of-sight access from the surface of the solid organic material in the enclosure. Particles can be ejected from the surface of the solid material when an electrical potential is applied to the housing to cause heat to be applied to the solid organic material in the enclosure causing the solid organic material to vaporize. By appropriate size and spacing of the baffle, the vaporized material and any ejected particles must first encounter at least one internal surface inside the source before exiting through the slit.
Improvement over the deposition source disclosed in commonly assigned U.S. Pat. No. 6,237,529 has been accomplished by replacing the slit in the top plate by an array of apertures, as disclosed in commonly assigned U.S. Patent Application Publication 2003/0168013 A1 by Dennis R. Freeman et al. In addition to overcoming nonuniformities associated with the slit geometry, Freeman et al. demonstrate the ability to compensate for end effects (i.e. the finite length of source and the consequent drop in relative deposition rate at the source ends) by varying the spacing between apertures (making them more closely spaced near the ends).
In addition, any nonuniformities in heat generation from the heater or heat absorption by the material to be deposited or distribution of said material within the source can give rise to nonuniformity in deposition along the length of the source. Yet another potential source of nonuniformity is unintended leaks in the source enclosure other than the apertures used to deliver the material vapor. If such leaks exist at the ends of the source, the flow of vapor from center to end of the source can cause pressure gradients within the source, thereby causing nonuniformity in the resultant deposition.
Forrest et al. (U.S. Pat. No. 6,337,102 B1) disclose a method for vaporizing organic materials and organic precursors and delivering them to a reactor vessel wherein the substrate is situated, and delivery of the vapors generated from solids or liquids is accomplished by use of carrier gases. In one embodiment of their invention, Forrest et al. locate the substrates within a suitably large reactor vessel, and the vapors carried thereto mix and react or condense on the substrate. Another embodiment of their invention is directed towards applications involving coating of large area substrates and putting several such deposition processes in serial fashion with one another. For this embodiment, Forrest et al. disclose the use of a gas curtain fed by a gas manifold (defined in the disclosure as “hollow tubes having a line of holes”) in order to form a continuous line of depositing material perpendicular to the direction of substrate travel.
The approach to vapor delivery as disclosed by Forrest et al. can be characterized as “remote vaporization” wherein a material is converted to vapor in an apparatus external to the deposition zone and more likely external to the deposition chamber. Organic vapors, alone or in combination with carrier gases, are conveyed into the deposition chamber and ultimately to the substrate surface. Great care must be taken using this approach to avoid unwanted condensation in the delivery lines by use of appropriate heating methods. This problem becomes even more critical when contemplating the use of inorganic materials that vaporize to the desired extent at substantially higher temperatures. Furthermore, the delivery of the vaporized material for coating large areas uniformly requires the use of gas manifolds. No mention of the requirements for such a gas manifold is made by Forrest et al.
As can be appreciated from the disclosure of Forrest et al., one skilled in the art would expect to have difficulty providing uniform films from an elongated source in which the material is vaporized along the length of a deposition source within the deposition zone.